The present invention relates to a method for exposing printing plates, in which light from a light source is imaged on a two-dimensional light modulator with a multiplicity of rows of light-modulating cells and is modulated by them, whereupon the light modulator is imaged via an imaging optical path onto light-sensitive material, wherein the light sensitive material is moved substantially perpendicularly to the direction of the rows of light-modulating cells relative to the light modulator at a relative speed, and wherein the data patterns to be imaged on the light-sensitive material, starting in the first utilized row of the light modulator, are each sequentially displayed in every row during a holding time, and subsequently are displaced to the particular following row of the light modulator until data patterns are displaced out of the last utilized row, with a shifting or displacement rate, through which the image of a given data pattern is, on average, kept stationary relative to the light sensitive material.
The invention also relates to an exposure and modulation arrangement for carrying out the method of the above described type, with a light source, a two-dimensional light modulator, with a multiplicity of rows of light-modulating cells, with a device for imaging on the light modulator, with a device for imaging the light modulator on light sensitive material, with a device for generating a relative movement between the light modulator and the light sensitive material, wherein the direction of the relative movement extends substantially perpendicularly to the direction of the rows of light-modulating cells, as well as with a device for displaying during a holding time, and subsequently displacing, a data pattern through the several columns of the light modulator, at a displacement rate, through which the image of a given data pattern is kept, on average, stationary relative to the light sensitive material during the movement.
The generic method and the generic arrangement are disclosed, for example, in DE 41 21 509 A1. Such arrangements and methods are of significance in particular for processes in which large quantities of modulated light in the blue and ultraviolet range are required, such as, for example, in the exposure of printing plates, the exposure of printed circuits and in stereolithography. The underlying principle is that the light sensitive material is continuously moved, while the image content is scrolled through the light modulator at like speed in the opposite direction. The image content thus remains nearly stationary on the material to be exposed. The exposure builds up through the integration of all short individual exposures of the cells of a row. In this manner strips with a width corresponding to the number of rows of the light modulator are exposed. By placing several strips next to one another, a larger area is exposed.
Problematic in this exposure principle is basically, on the one hand, the extremely large data quantity, which must be transmitted for each exposure step. The data transmission rate between an activating electronic circuitry and the light modulator is herein limited by nature. As the light modulators are utilized, for example, two-dimensional micromirror rows (DMD™), liquid crystal displays, diffraction gratings, light valves or reflecting liquid crystal lines (LCOS). If the printing plates to be exposed are sufficiently sensitive and/or if the radiative flow provided by the light source has reached a certain magnitude, the throughput of a generic arrangement, measured in numbers of exposed printing plates per hour or in exposed square meters per hour, is limited by the data rate at which data can be transmitted to the display. The data rate in the conventional generic scrolling methods is limiting for the throughput, since the complete light modulator must be written anew every time the image is advanced by one row.
In order to increase the throughput nevertheless, DE 10 2005 015 193 A1, for example, has proposed decreasing the number of lines actually used by the light modulator and activated compared to the number of physically available lines. However, of disadvantage herein is that the range of the system is decreased which, in turn, decreases the magnitude of the radiative flow.
A further problem in conventional scrolling methods of the above described type is that the relative movement between the light modulator and the light sensitive material is a continuous movement, whereas the movement of the data pattern on the light modulator is an abrupt movement. The latter can be traced back to the fact that the movement, thus the scrolling, of the data pattern across the light modulator can only occur in discrete steps, wherein the step width is predetermined by the number of rows and the width of the light modulating cells. The data patterns thus do not move continuously across the light modulator, but rather are, instead, displayed in a row during a holding time, during which the data pattern does not move and subsequently jumps quasi abruptly to the adjacent row. Consequently, the image of a given data pattern can only be kept, on average, stationary relative to the light sensitive material. However, the image of the data pattern, in fact, moves during the holding time on the light sensitive material at a rate corresponding to the relative speed between the light modulator and the light sensitive material. This movement of the image of the data pattern during the holding times leads, according to prior art without further measures, to the fuzziness, as a consequence of the principle, of the exposure of the width of a light modulating cell of the light modulator.
DE 41 21 509 A1 therefore proposed that the image of the light modulator on the material to be exposed is reduced by means of a lens, such that the exposed area on the material to be exposed due to the blurring in the end result receives again the original size. However, of disadvantage herein is that the layout of the reducing optical system can under certain circumstances be problematic, in particular if the relative speed between the material to be exposed and the light modulator as well as the holding time should or must be varied. In principle, in these cases the reducing lens would have to be adapted to the new parameters. However, this is disadvantageously only possible with comparatively complex and expensive additional optical components.